@RoosGirl
He can't. It's Congress that makes those decisions.
Congress has said nothing about transgender troops -- the law is the same as it has always been. The policy change came from President Obama through the civilian Secretaries, and could be reversed the same way.
I just think that they want to minimize the political firestorm by building a "case" for reversing some of those changes before actually doing so. It will be infinitely easier for Mattis, et al, to say "we tried it, and this is what we found", rather than being accused of simply speculating as to future problems.
Maybe I'm wrong, and giving them too much of the benefit of the doubt. But I recall back when the women in combat thing first started, and the Marines were the
first to come up with integration plans and testing. People accused them of selling out, but in fact, they were just building a case so that when it came time to recommend against it, they'd have solid facts. And as it turned out, the Marines were the only branch to formally request that they
not be required to integrate women, and then offered their studies as evidence. Mabus overruled the objection, but the Corps' leadership strategy of appearing to give the idea a chance before rejecting it was sound.
Frankly, I think the best time to do that again would be next summer -- make it an election issue as to whether or not Democrats will openly oppose a recommendation from the very popular SecDef based on evidence. Trust me, Mattis will have all his ducks lined up. They'll have stats for the costs of transgender troops, number of missed days/deployments, medical issues, etc.. And he'll lay out a compelling factual case why it is a tremendous waste of resources. Then let Dems running for Congress try to make trannies in the Marines (and elsewhere) the issue on which they want to hang their hats.
That is much less popular with most Americans than it is with the fringe.